Gaseous precipitation of metals from solution



r GASEOUS PRECIPITATION F METALS FROM SOLUTION Patrick J. McG-auley, Glen Cove, N. Y., assignor to American Cyanamid Company, New York, N..Y., a corporation of Maine i Application October 19, 1956, 4 Serial No. 616,941

5 Claims. (Cl. 75-.5) 7

- No Drawing.

This invention is concerned with ajprocess for the precipitation of non-ferrous metals from aqueous solutions of dissolved metal salts by theaction of reducing gases at elevated temperatures and superatmospheric pressures. More particularly, it relates to an improvement in such a process to obtain commercially useful copper, cobalt and nickel as free metal powder of more desirable physical-and chemical characteristics in higher yield and in shorter time.

As such, this application is a continuation-in-part of my co-pending application for United States Letters Patent, Serial No. 290,557, filed May 28, 1952, now abandoned.

In recent years, there has been a continually increasing commercial interest in the hydrometallurgical recovery of non-ferrous metals, particularly copper, nickel and cobalt, from various ores, ore concentrates, plant by-products and secondary metals. Various improvement proposals have been made, not only in leaching methods, but also in methods of separately or conjointly precipitating metal powders from solution and in overall processes combining these features. It is with the precipitation of elemental metals in powder form from solutions of salts of the metals with which this present invention is concerned.

Prior to my present invention, it has been proposed to treat metal salt solutions under reducing conditions to precipitate metal contents therefrom. In general, such proposals were intended to serve one of two purposes. The first was to separate a plurality of metals in solution from each other as an alternative to fractional recrystallization. The second was to prepare a supported metal catalyst by precipitating the metal content on kieselguhr or other suitable supporting material. The primary object in the first case was to obtain a separation of the dissolved contents into products which could then be otherwise treated to produce commercially desirable metal products. In the second case, the sole object was to deposit on the support some metal-bearing product which could and would be further reduced to metal in the conventional heating in a reducing atmosphere used to activate a supported metal catalyst.

In neither case was the physical form of the precipitated product of any particular concern. The same was generally true of the chemical nature of the product. The primary interest was to precipitate the desired dissolved metal content in some form. Therefore, in both cases, it was usual for the metal content to be precipitated as a metal salt, or as an oxide, hydroxide, basic salt or the like, or as mixtures thereof. In some cases, the metal content was precipitated in part as free metal. However, the actual metal content was wholly incidental, was not controllable, often was only aninsignificant part and. always was so contaminated with metal compounds "of the types noted as to have no commercial value as an elemental metal product without further treatment even if the object of such processes had been to produce such a product.

" Some attempts were also made to redissolve the impure metal products and reprecipitate them by disproportionation in more nearlypure form. Unfortunately, this always reduced the potential yield and was never commercially successful in .producing the desired purity.

In contrast therewith, the present invention is concerned with precipitating free metal powder products in sutficiently pure form to meet the present standards for Since it involves an improve-' acceptable metal powder. ment on processes for obtaining such a product by obtaining them substantially free of contaminating metal com.-, pounds, the above-described proposals are of no particular concern herein.

In contrast to these proposals is the invention described in U. S. Patent No. 2,734,821. In general, this patent teaches that elemental metal, free of by-product im purities, can be precipitated ,from a metal salt solution 1 by strictly observing a combination of fac'torsI First; strict control is made of (1) the composition of the solu tion,'i. e., its dissolved metal concentration, its content of free hydrogen or hydroxyl ions, ions or salts capable of forming a soluble complex of the metal, and extraneous. dissolved materials; (2) the reduction temperature; (3)" the reducing gas partial pressure" and the total pressuref and (4) the use of agitation. Second, solution composi-' tion is controlled not only at the outset but throughout the entire reduction period. Of primary importance is the initial adjustment of operating conditions accom panied by subsequent adjustments if and when necessary, to prevent the occurrence of certain terminal conditions until metal precipitation has reached asatisfactory stage. 'As notable as is the process of US. Patent No.

2,734,821, it, nevertheless, suffers from certain draw-' backs. Principally, it has been found that when practicing" said process, particularly under industrial conditions,

much of the precipitated metal content deposits on and adheres to the lining and agitating means of the reaction I The conse-'. quential lowering in yield of product powder is not in-' vessel and/or forms in the vessel as foil.

signficant nor is the loss sufi'eredby the frequent shutdowns necessitated for removing adhered metal by mechanical 'and/ or chemical means. In addition, extended reaction times are required to obtain the product powder" and the latter is sometimes irregular in shape and of low apparent density.

' The present invention is in a method for improving such reduction processes as that of U. S. Patent No. 2,734,821, particularly with respect to the recovery of' copper, nickel and cobalt so as to overcome the various drawbacks noted.

It is the principal object of this invention to provide such a method. An important object is to minimize adherence or plating of precipitated metal powder to the walls and agitating means of the reduction vessel. A

further object is to recover copper, nickel and cobalt in discrete form of a suitable uniform particle size and These various objects have been met in a simple yet completely effective manner. In general, the processof the invention follows the process of U. S. Patent No.

2,734,821. The improvement resides in conducting the reduction process in the presenceof metal powder or seed slurried in the solution undergoing treatment.

Prior to a more detailed description of the invention, certain terms unique to the art will be defined so as to minimize the possibility ofconfusion. Accordingly, the

following terms: used throughout the specification and;

claims will have the following meanings:

' Patented Sept; 3,1957

The metal powder or. seedemployed in the densification reduction process of this invention may be any suitable metal powder but will usually be of the metal to be precipitated. Seed powder from any source is satisfactory, provided it has certain characteristics to be subsequently discussed. Probably the least desirable source is to mechanically size-reduce metal powder obtained by densification reduction. Another source of seed is. disclosed in U. S. Patent No. 2,753,257 which discloses a method for treating a solution under conditions which are quite restrictive but optimum for self-nucleation. Resultant nuclei may then be used as seed when treating a solution under conditions optimum for densification. Preferably, however, the most desirable source of seed involves a method which comprises subjecting a metal salt solution to gaseous reduction in the presence of any of numerous nucleation promoters as disclosed in US. Patent Nos. 2,767,081, 2,767,082 and 2,767,083. Nuclei so produced may then be employed as seed in densification of the nucleation solution and/ or other portions of solution.

However produced, size, bulk density and activity are seed characteristicsthat must be considered. Bulk density is a measure of surface irregularity. Powder consisting of spherical grains of small surface area may have a bulk density as high as about 4 grams per cubic centimeter, whereas powder which is dendritic or which consists of loosely cemented aggregates of finer particles and therefor a large surface area may have a density as low as 0.4. During the densification reduction reaction, the slurried metalpowder seed is built up by deposition of precipitated metal on its surfaces. Therefore, powders with high surface area per unit weight are most effective. Since such powders are also of low density, the necessary amount of powder is better measured asvolume of powder rather than weight of powder. Effectiveness and activity of the powder depend not only on surface area, but also on the nature of the surface. Oxide or other films,

for example, will slow the reaction untilremoved by reduction or leaching.

Metal seed powder used should be finer than 20 mesh and preferably finer than 150 mesh. Fineness is particularly important when the initial particles are spherical and have a high bulk density. As little as about 0.5 to 5 ml. of-seed for each grams of dissolved metal per liter of solution, depending on its fineness, surface area and activity, noticeably reduces plating and increases the reaction rate, the latter particularly with respect to nickel and cobalt. Still further improvement is obtained using quantities up to about 150 ml./liter. More than about 150 ml. may begin to show disadvantages because of the power increase required for agitation, and may show a decrease in aifect because of inability tokeep the seed in suspension. The weight of such quantities of seed powder will generally range from about 5-100-grams/ liter depending on the density, which preferably is less than 1.0.

Purity .of seedpowder is important. In order for the subsequent metal powder product. to meet commercial specifications, the seed powderrnust not contain so much oflany. impurity so that when diluted with the newly (i6: posited metal the resulting m ixture will exceed the limit for that impurity imppsed by. suchspecifications.

the higher the proportion of seed powder used per unit of newmetal obtained, the greaterthe requirement for purity in the seed powder.

During the reduction reaction, the seed is maintained in suspension by adequate agitation. After optimum reduction of a batch of solution is reached, the solution is discharged and a new batch treated. It is possible, according to this. invention, to conduct as many as 15-20 and even as high as 40 densification reductions using the same seed powder. On successive use, the seed particles becomedense and grow in size assuming a generally spherical shape and uniformity in size. As the densifications proceed, and the total surface area of the seed becomeslarger, the effective surface area becomes smaller because of the increased portion of'seed settling to the bottom of the reaction vessel. When .the size-of the particles has increased to the extent where it is difiicult to hold them in suspension, the powder is discharged and the procedure repeated with fresh seed. r

In general, the specific nature or origin of the solu'on to be seeded maybe consideredas independent of the present invention. Usually the solution is aqueous and will have been obtained by some known per se leaching operation of a suitable ore, ore concentrate, metallurgical plant by-product or secondary metals.

The metals. which may be treated according to the prescut invention may be any non-ferrous metal having an oxidation-reduction potential between those of cadmium and silver, inclusive, and which is capable of forming with ammonia a complex ion that may be reduced to elemental metal with a suitable reducing gas. Practically, this contemplates siiver, mercury, copper, nickel, cobalt and cadmium, but the metals principally encountered will be,

copper, cobalt and nickel. Copper, however, is more easily reduced under a wider range of conditions, and therefore, at least so far as increasing the rate of reduction is concerned, the need for seed is not so great as with cobalt and nickel. The non-metallic ion which goes to make up the metal salt to be reduced may be of any inorganic acid or strong organic acid providedit forms a soluble salt of the metal and is not reduced under precipitation conditions. Generally these will be limited to the chlorides, acetates, sulfates, and carbonates. The nitratesare good under basic conditions but are not usefulunder acidic reduction conditions. In. actual practice, only the sulfates and carbonatesare commonly encountered. Theinvention has its greatest applicability in the treatment of copper, nickel and cobalt sulfates whether originally obtained by dissolving salts or by acid or ammoniacal leaching. Particularly when using ammoniacal leaching as a method of preparing the solution, it will be found that the solutions will contain varying amounts of ammonium sulfate or ammonium carbonate and it is contemplated that one or both of these materials may be and usually are present.

Depending upon solution conditions, the dissolved metal salt may be in any one of several forms. For example, in pregnant leach liquors the ammonium salt content may be quite high and the metal salt may be a simple salt such as the metal sulfate-MeSOr or a complex metal ammine saltsuch as a metal ammine sulfate Me(NHs)rSO4 in which x may vary about 1.5-6.0. The former is usually that found to some extent in solutions having an acidity greater than about pH 4-5. The latter may be found in acid solutions but appears in appreciable amounts at hydrogen ion concentrations less than aboutpH 5.5 in the case of cobalt or about pH 6.5 in thecaseofcopper and nickel. The ammine salt will be found to comprisesubstantially the entire dissolved metal content when theacidit'y is less than a pH of about 7.5. Both forms may be found in the intermediate- Between plivalues of about 5-5.5 for cobalt,

ranges. and 5-6.5 for copper and nickel, metal hydroxides and/ or basic salts may befoundwhen theammonium sulfate content is low. In some cases, in solutions of near neu tral or slightly acid characteristics, the dissolved metal may form a double salt such as a metal sulfate-ammonium sulfate double salt, probably of the structure M6804 (NH4) 2804 XHzO as the solution is cooled. This will not be commonly encountered in leach liquors because, although the double salt is rather soluble as compared with the other forms, in the presence of a suflicient amount of ammonium salt it is practically insoluble. In solutions containing considerable excess free acid, particularly in the case of sulfates, some form of acid complex with the free acid appears to form. However, these sulfate complexes or their analogues are reducible under the conditions ofthe present invention.

The total concentration of metal in'solution may be as high as 200 g./l. depending upon the metal, the anion and whether the solution is acidic or ammoniacal. However, it should be less than that at which in the absence of a partial pressure of reducing gas, a substantial amount of a, compound of said metal will precipitate on heating to-the reduction temperature, i. e., above about 250 F. A typical feed solution may be considered as an aqueous sulfuric acid or aqueous ammoniacal solution containing dissolved salts, illustratively sulfates, of copper, nickel, cobalt and ammonia. rated and ammoniacal, may contain 150 grams per liter vof dissolved metal particularly in the case of copper.

Acidic solutions usually run lower, about 100 grams per liter. In some cases these figures may be higher, in others somewhat lower. A solution containing less than about 5-10 grams per-liter is ordinarily uneconomical to treat.

commercially without first using some means of concentrating the solutes content. A typical solution of nickel and cobalt sulfates found in practice will usually contain from about 30-70 grams per liter of combined nickel plus cobalt. For commonly encountered copper sulfate and copper carbonate solutions, the copper content will generally not be more than about 100 g./l. and 135 g./l.,

lization of any dissolved compound. This necessitates its being compatible withall the dissolved components.

In the case of sulfate solutions, sulfuric acid itself orsaltsformed in neutralizing an excess of it such as sodium, potassium and ammonium sulfates may serve the purpose; Any of such would be a compatible sulfate.

The compatible electrolyte is usually the ammonium salt of the anion in solution. ammonium salt present in the adjusted solution vary as the metal and anion, and in the present invention are the same as taught in U. S. Patent No. 2,734,821. Illustratively, in sulfate solutions the ammonium sulfate content will usually be-a-bout 0.1-0.5 M/l. for copper and about 0.5-1.5 M/l. for nickel and cobalt.

The effect of hydrogen ion concentration on reduction varies with the metal being treated. Copper may be precipitated from solution under quite wide hydrogen ion concentrations, the upper limit appearing to be about equivalent to a 20% H2804 solution. For nickel and cobalt, the limits appear to be about 5% and 0.5% respectively. For optimum results, the hydrogen ion concentration should be initially adjusted to considerably less than these limits and maintained throughout the reduction by the presence of a material such as ammonia.

Densification is carried out using a suitable, sulfur-free reducing gas such as carbon monoxide, hydrogen or a Such a sulfate solution, if satu- The preferred amounts of' 6 combination of them. In general, hydrogen is preferable. Densification is carried out at a temperature above J about 250 F., usually between about 250-550" F. At least sutficient total pressure, including the reducing gas overpressure, is used to prevent boiling of the solution at the reaction temperature. Usually the total pressure will not be greater than 1000 p. s. i. g. While reduction may beaccomplished in some instances with a reducing gas partial pressure of as little-as one atmosphere, it preferably is at least about 50 p. s. i. g. and usually from'300-500 p. s.- i. g. or even higher. a

The following examples demonstrate the present invention and are meant to be illustrative'only and not by wa 4 of limitation.

Example 1 An aqueous ammoniacal solution containing about grams of nickel as nickel sulfate, about 200 grams per liter of ammonium sulfate and about 35 grams NH3 per liter of solution is charged to an autoclave with-minus 150 mesh nickel seed powder, the latter in the amount of 150' ml. per liter of solution. The slurry is continually stirred and heated to about 400 F. Hydrogen is then admitted under pressure until the total pressure is about 900 p. s. i.,-

' the partial pressure of hydrogen being about 400 p. s. i.

When a series of runs is conducted in which the pressureand temperature as-defined are maintained for different periods of time varying from 10 minutes to 60 minutes ac companied by constant stirring, it is found that in about 10-30 minutes the recovery of nickel is highly acceptable.

Up to 99% of highly pure nickel of uniform fine particle Example 1 is repeated, substituting about 80 grams of cobalt sulfate for the nickel sulfate, a like amount of cobalt seed for the nickel seed and a temperature of 450 C. Recovery of up to 99% of highly pure cobalt of uniform fine particle size is obtained in from about 15-30 minutes.

'3 Example 4 A solution obtained from a previous leaching operation.

and containing about 70 grams of copper as copper sulfate and about 75 grams of NH3 per liter of leach solution is introduced into an autoclave. 75 ml./l. of copper seed powder of minus 150 mesh is then added, the slurry heated to 325 F. and hydrogen introduced until the total pressure is 600 p. s. i., of which the hydrogen partial pressure is about 300 p. s. i. After 30 minutes with con- Recovery of up to 99% of copper of uniform fine particle size is obtained.

Example 5 A cobaltous sulfate solution containing 50 grams per liter of cobalt, 0.8 mol (NH4)2SO4 and 3 mols NH3 per mol of cobalt is heated to 425 F. and subjected-to a hydrogen partial pressure of 400 p. s. i. After 2 hours treatment a sample taken shows less than 10% reduction of the cobalt. Treatment is continued for another hour and over reduction is obtained but the cobalt powder produced is very fine and is mostly found on the Walls of the autoclave as a soft spongy deposit.

Example 6 The procedure of Example 5 is repeated except that ml. per liter of solution of the nickel seed powder of Example 1 is used. A sample taken after 30 minutes shows 95 reduction and when treatment is stopped after one; hour reduction isover 99% complete. The: metal powder is fine and light with little adherence to the walls of the-autoclave;

Treatment of acid solutions is similar to that ofbasic solutions. However, the metal complexions are probably of the type M6(AC);uwherein Me again represents the metal, Ac the acidic anion and x a small integer. The negative charges depend on both Me andAc. Formation thereof is influenced by the concentration of-the acid anion which; is controlled by additioned or formationof the acid or its salts with ammonia or alkali metals. The present inventionis of assistance in this type of reduction also. Theproductrismore uniform insize and=quality and in general can be taken in higher yield, in, shorter time and at lower temperatures. This is illustrated in the following examples.

Example 7' An aqueoussulfuric acid solution containing about 55 grams per liter of nickel as nickelsulfate; about 5 grams per; liter of H2504 and about40 grams per liter of (NI-D2504 is charged to an autoclave and heatedto 400 F, Hydrogen is then introduced until the total pressure l-750 p. s. i., the partial pressureof hydrogen being about 400 p. s. i. NH3 is introduced in suiiicient amount to maintain the acidity at approximately 0.5% acid, keep ing an appreciable concentration of sulfate ions to prevent their conversion to bisultates. After 30-.minutes, the pressure is released and the precipitate collected. When dried ina reducing atmosphere, a yield of about 90% nickel is obtained. The nickel product hasga fluffy and spongy appearance and adheres in partto the walls of the vessel.

Example 8 Example 6 is repeated except that about 50ml. per liter of minus 150 mesh nickel powder is charged; along with the solution to the autoclave. A yield of about 99% nickel is obtained without plating on the vessel walls. The product is of more uniform particle size and not spongy as in Example 7.

The above examples all demonstrate'the minimizing.

An ammoniacal sulfate solution containing 50 g./l. nickel as nickel sulfate, an NHszNi mol ratio of 3:1 and an (NH4)2SO4:Ni mol ratio of 1:1 is subject to nucleation reduction in an autoclave at 350 F. with hydrogen at a total pressure of 600 p. s. i. g. inthe presence of 3 g./l. of Fe as ferrous sulfate. The reaction is con: tinned until the nickel content of thesolution is reduced to 1 g./l. During the reaction, finely dividednickel nuclei or seed particles of about 1 micron are formed.

Example The slurry of Example 9 is permitted to settle and the liquor drawn off. A second charge of the solution of Example 9 free of ferrous ions is introduced into the 8 autoclave and subjected to densificatiorr reduction: under similar conditions in the presence of the: seed formed in Example9. The seed provides a large area for the. newly precipitated nickelto deposit as athinskin. When the nickel content is reduced to about 1 g./l., the reduction is stopped, liquor drawn ofi and'a fresh batch of pregnant solution introduced. Densification is thus repeated until the nickel particles have grown to about microns, the total'number of densification reductions being about 40.

I claim:

1. In precipitating a substantially oxide-free. elemental metal powder selected from the group consisting of copper, nickel and cobalt'by treating a solution of a disc solved salt. of at least one such metal with a. non-sulfidizing reducing gas at an elevated temperature and a superatmospheric pressure, the method for minimizing adherence of precipitatedmetal to the surfaces, of'the vessel in which said treatment is conducted and for recovering precipitated metal as av powder of uniform particle size in increased yield and purity at a faster reaction rate which comprises: adjusting such a solution to contain (a) atotal concentration of said metal up to about 200 g./l., but less than that at which, in the absence of a partial pressure of'reducinggas, a substantial amountof'a. compound of said metal will precipitate while heating to above about 250 F., (b) at least a part of said dissolved metal in ajorm of a solublegas-reducible ion, (c) a hydrogen ion concentration up to. about that equivalent to a 20% free sulfuric acid solution but less than that at which precipitation of said dissolved metal in a form other than elemental metal powder occurs under reduction conditions and (d) a finite amount of a compatible electrolyte,

treating said adjusted solution to an overpressure of'a non-sulfidizing reducing gasof at least about 50 p s. i. 'g while maintaining. a total pressure sulficient' to prevent boiling while maintainingsuspended in said solution at leastahout 0.5 ml. of seed metal powder for about each 10 grams of dissolved metal per liter of solution, said seed metal powder being all substantially minus 20 'mesh; and providing a material capable of reacting with acidic anions to control the hydrogen ion concentration so as to precipitate elemental metal only, whereby said seed metal powderis densified by'the deposition thereon of precipitated elemental metal powder, continuing treatment until optimum precipitation is-complete, and treating additional solution in the samemanner in the presence of the sameseedpowder.

2. The method according to claim 1 in which the-metal is copper.

3. The method according to'claim 1 in which the metal is nickel.

4. The method according-to claim 1 in which the metal is cobalt.

5. The method according to claim l in which the amount of seed-metal powder is about -5-100'grams 'per liter of solution.

References Citedin the file of this patent UNITED STATES PATENTS.

1,783,662 Marx .,Dec. 2, 1930 2,734,821 Schaufelberger Feb. 14, 1956 

1. IN PRECIPTATING A SUBSTANTIALLY OXIDE-FREE ELEMENTAL METAL POWDER SELECTED FROM THE GROUP CONSISTING OF COPPER, NICKEL AND COBALT BY TREATING A SOLUTION OF A DISSOLVED SALT OF AT LEAST ONE SUCH METAL WITH A NON-SULFIDIZING REDUCING GAS AT AN ELENATED TEMPERATURE AND A SUPERATMOSPHERIC PRESSURE, THE METHOD FOR MINIMIZING ADHERENCE OF PRECIPITATED METAL TO THE SURFACES OF THE VESSEL IN WHICH SAID TREATMENT IS CONDUCTED AND FOR RECOVERUNG PRECIPITATED METAL AS A POWDER OF UNIFORM PARTICLE SIZE IN INCREASED YEILD AND PURITY AT A FASTER REACTION RATE WHICH COMPRISES: ADJUSTING SUCH A SOLUTION TO CONTAIN (A) A TOTAL CONCENTRATION OF SAID METAL UP TO ABOUT 200 G/L BUT LESS THAN THAT AT WHICH, IN TH EABSENCE OF A PARTIAL PRESSURE OF REDUCING GAS, A SUBSTANTIAL AMOUNT OF A COMPOUND OF SAID METAL WILL PERICIPITATE WHILE HEATING TO ABOVE ABOUT 225*F., (B) AT LEAST PART OF SAID DISSOLVED METAL IN A FORM OF A SOLUBLE GAS-REDUCIBLE ION, (C) A HYDROGEN ION CONCENTRATION UP TO ABOUT THAT EQUIVALENT TO A 20% FREE SULFURIC ACID SOLUTION BUT LESS THAN THAT AT WHICH PRECIPITATION OF SAID DISSOLVED METAL IN A FORM OTHER THAN ELEMENTAL METAL POWDER OCCOURS UNDER REDUCTION CONDITIONS AND (D) A FINITE AMOUNT OF A COMPATIBLE ELECTROLYTE, TREATING SAID ADJUSTED SOLUSTION TO AN OVERPRESSUSRE OF A NON-SULSFIDIZING REDUCING GAS OF AT LEAST ABOUT 50 P.S.G. WHILE MAINTAINING A TOTAL PRESSUSRE SUFFICIENT TO PREVENT BOILING WHILE MAINTAINING SUSPENDED IN SAID TO PREVENT LEAST ABOUT 0.5 ML. OF SEED METAL POWDER FOR ABOUT EACH 10 GRAMS OF DISSOLVED METAL PER LITER OF SOLUTION, SAID SEED METAL POWDER BEING ALL SUBSTANTIALLY MINUS 20 MESH; AND PROVIDING A MATERIAL CAPABLE OF REACTTING WITH ACIDIC ANIONS TO CONTROL THE HYDROGEN ION CONCENTRATION SO AS TO PRECIPITATE ELEMENTAL METAL ONLY, WHEREBY SAID SEED METAL POWDER IS DENSIFIED BY THE DEPOSITION THEREON OF PRECIPITATED ELEMENTAL METAL POWDER, CONTINUING TREATMENT UNTIL OPTIMUM PRECIPITATION IS COMPLETE, AND TREATING ADDITIONAL SOLUTION IN THE SAME MANNER IN THE PRESENCE OF THE SAME SEED POWDER. 